1. Field of the Invention
The invention relates to improved cermet electrodes and to a method for preparing such electrodes. The invention has specific application in the production of anodes for the electrowinning of aluminum in Hall-Heroult cells.
2. Description of the Prior Art
Electrolysis cells, such as a Hall-Heroult cell for aluminum production by the electrolysis of alumina in molten cryolite, conventionally employ conductive carbon electrodes. During the reaction to manufacture aluminum metal, the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##EQU1##
The problems caused by the use of carbon anodes are related to the cost of the anode consumed in the above reaction and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the fabrication of the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing troublesome workplace and environmental pollution problems. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon were consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered. More importantly, with no carbon consumed at the anode there would be no contamination of the atmosphere or the product from the impurities present in the coke.
Attempts have been made in the past to use non-consumable electrodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen or by the cryolite bath. Ceramic compounds such as oxides with perovskite and spinel crystal structures usually have too high electrical resistance or are attacked by the cryolite bath. Electrodes consisting of metals coated with ceramics using conventional methods such as thermal spraying have also shown poor performance, in that almost inevitably, even the smallest crack leads to attack on the metal substrate by the molten salt bath.
Recently, efforts have been made to fabricate non-consumable electrodes from special compositions known as cermets. A cermet composition is defined as one consisting of both metallic and ceramic phases. The conventional method of preparing cermet compositions is to mix metal and ceramic powders, cold press a preform, and sinter the preform at an elevated temperature in a controlled atmosphere. Alternatively, the cermet may be prepared by hot pressing or hot isostatic pressing (HIP) wherein the sintering operation is carried out under pressure. Cermets have high electrical conductivity in comparison to ceramic compositions and good corrosion resistance when compared to metals.
U.S. Pat. No. 4,374,050 to Ray provides a non-consumable electrode for molten salt electrolysis fabricated from at least two metals or metal compounds combined to provide a combination metal compound containing at least one of the group consisting of oxide, fluoride, nitride, sulfide, carbide or boride, the combination metal compound defined by the formula: ##EQU2## where Z is a number in the range of 1.0 to 2.2; K is a number in the range of 2.0 to 4.4; M.sub.i is at least one metal having a valence of 1, 2, 3, 4 or 5 and is the same metal or metals wherever M.sub.i is used in the composition; M.sub.j is a metal having a valence of 2, 3 or 4; X.sub.r is at least one of the elements from the group consisting of O, F, N, S, C and B; m, p and n are the number components which comprise M.sub.i, M.sub.j and X.sub.r ; F.sub.M.sbsb.i, F'.sub.M.sbsb.i, F'.sub.M.sbsb.i or F.sub.x.sbsb.r are the mole fractions of M.sub.i, M.sub.j and X.sub.r and 0&lt;.SIGMA.F'.sub.M.sbsb.i &lt;1.
U.S. Pat. No. 4,374,761--Ray relates to non-consumable electrodes for molten salt electrolysis comprised of a ceramic oxide composition and at least one metal powder dispersed through the ceramic oxide composition for purposes of increasing its conductivity, the metal powder selected from the group consisting of Ni, Cu, Co, Pt, Rh, In and Ir.
U.S. Pat. No. 4,397,729 to Duruz et al. discloses a non-consumable anode for molten salt electrolysis consisting of a cermet material formed from a ceramic oxide of, e.g., a ferrite or chromite, and a metal, e.g., a noble metal or alloy thereof.
U.K. Pat. Appln. No. 2,069,529A to Duruz et al. provides a non-consumable anode for molten salt electrolysis consisting of a cermet material comprising at least one ceramic oxide such as chromite or ferrite of iron or nickel or ferric or chromic oxide and at least one metal such as nickel or chromium or a noble metal, e.g., palladium, or an alloy of such metals.
One of the problems arising in the development of conductive ceramic and cermet electrodes has been caused by the difficulty of making a durable electrical connection between the electrode material and the current conductor. Previous efforts in the field have produced connectors, primarily of metals such as silver, copper, and stainless steel. Can, U.S. Pat. No. 3,681,506, discloses a resilient metal washer held in place to form an electrical connection. Davies, U.S. Pat. No. 3,893,821, discloses a contact material containing Ag, La, SrCrO.sub.3 and CdO. Douglas et al., U.S. Pat. No. 3,922,236, disclose a contact material containing Ag, Cu, La, and SrCrO.sub.3. Fletcher, U.S. Pat. No. 3,990,860, discloses cermet compositions containing stainless steel or Mo in a matrix of Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3. Shida et al., U.S. Pat. No. 4,141,727, disclose contacts of Ag, Bi.sub.2 O.sub.3, SnO.sub.2 and Sn. Schirnig et al., U.S. Pat. No. 4,247,381 disclose an electrode useful for AlCl.sub.3 electrolysis comprising a graphite pipe, a metallic conductor with a melting point below the bath temperature, and a protective ceramic pipe surrounding the former. West German No. 1,244,343, discloses borides or carbides of Ti, Zr, Ta, or Nb cast to Al using a flux of Li.sub.3 AlF.sub.6, Na.sub.3 AlF.sub.6 and NaCl. Alder, U.S. Pat. No. 4,357,226, discloses an anode assembly for a Hall cell comprising individual units mechanically held together by a clamping arrangement.
Pending U.S. patent application Ser. No. 475,951 to Secrist et al., now U.S. Pat. No. 4,443,314, relates to an improved anode assembly for an aluminum electrolysis cell formed by the process of shaping an anode and a cermet connector from powders, machining the articles, and then sintering the articles. The cermet connector mates with the anode via a threaded joint located at its region of high temperature during operation thereof to avoid excessive ohmic losses. Mechanical support can be provided by the threaded joint or through the use of separate mechanical suspension bars.
Pending U.S. patent application Ser. No. 491,089 to Secrist et al., now U.S. Pat. No. 4,472,258 provides an electrode for an electrochemical cell comprising a variable cermet composition, the portion in contact with the electrolyte having a relatively high ceramic content for maximum corrosion resistance and the portion attached to the external electrical circuit having a relatively high metal content to facilitate an electrical connection. Preferred metals are Ni, Cu, Fe, and Cr; and preferred ceramics are ferrites.